Oil production vessel

ABSTRACT

A ship is described which is useful for recovery of fluid minerals from the ocean floor. A rotatable mooring swivel through an aperture in the ship&#39;&#39;s hull near the bow is moored to the sea floor. A riser pipe from the ocean floor passes through the center of the mooring swivel and is connected to a substantially vertically extending tower above the ship. This permits the ship to &#39;&#39;&#39;&#39;weathervane&#39;&#39;&#39;&#39; about the moored plug and riser. The tower rotates with the ship and a fluid swivel is provided between the upper end of the riser and the tower to accommodate relative rotation. In addition, vertical translation of the riser upper end relative to the length of the tower is permitted and the tower is mounted on gimbals for tilting relative to the deck of the ship to remain aligned with the riser end.

United States Patent [72] Inventors George W. Morgan [73] Assignee NorthAmerican Rockwell Corporation [54] OIL PRODUCTION VESSEL 3,191,2016/1965 Richardsonetal. ll4/0.5(D)X 3,390,654 7/1968 Bromell et al. Il4/0.5 (D) 3,407,768 10/1968 Graham 1 14/230 X 3,437,064 4/1969Hunsucker .7 1 14/05 D 3,440,671 4/1969 Smulders 1 14/230 X PrimaryExaminer-Trygve M. Blix Attorneys-William R. Lane and Allan RothenbergABSTRACT: A ship is described which is useful for recovery of fluidminerals from the ocean floor. A rotatable mooring swivel through anaperture in the ships hull near the bow is moored to the sea floor. Ariser pipe from the ocean floor passes through the center of the mooringswivel and is connected to a substantially vertically extending towerabove the ship. This permits the ship to "weathervane" about the mooredplug and riser. The tower rotates with the ship and a fluid swivel isprovided between the upper end of the riser and the tower to accommodaterelative rotation. In addition, vertical translation of the riser upperend relative to the length of the tower is permitted and the tower ismounted on gimbals for tilting relative to the deck of the ship toremain aligned with the riser end,

PATENTEU AUGSI t97| 3.602175 sum 1 ur 3 INVENTORS GEORGf W. MORGAN BRUNOR. NACZKOWSKI O ZQQMMQQg A TTORNE Y PATENTEUAUG31 Ian 3,602,175

sum 2 or 3 INVENTORS. GEORGE W MORGAN BRUNO R, NACZKOWSK/ A T TORNE YOIL PRODUCTION VESSEL BACKGROUND In the offshore production of oil andother subaqueous minerals there is a practical depth limitation forbottom mounted towers that extend above the surface. This practicallimit is in the order of about 600 feet and economic considerations maylimit the utility of towers to even shallower depths. An alternative tofixed towers is to provide a portion of the production facilities on thesea floor and provide a permanently moored floating facility for thebalance. When this is done a substantially vertically extending conduitor riser must be provided between the sea floor and the floatingfacility. In relatively shallow depths and calm waters many of theproblems of permanently interconnecting a floating facility and the seafloor have been solved. However, when the depths become great thestresses involved in a moored ship and fixed riser system becomesignificant and when a permanent mooring is considered, the ability towithstand storm conditions must be provided.

SUMMARY OF THE INVENTION There is therefore provided in practice of thisinvention a seagoing tankerlike vessel having a rotatable plug ormooring swivel therethrough near the bow for mooring to the bottom andaccommodating a riser passing therethrough. A tower mounted over theplug supports a portion of the upper end of the riser and provides formotion of the riser end along the length of the tower and relative tiltof the tanker relative to the riser in two directions. Fluid swivelmeans are provided for accommodating rotation between the ship and theriser. Means are further provided for retaining the ship permanently inposition during heavy seas without damage to the riser.

DRAWINGS Objects and many of the attendant advantages of this inven tionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 illustrates in perspective a ship constructed according to theprinciples of this invention;

FIG. 2 illustrates in perspective cutaway a mooring swivel and risertower on the ship;

FIG. 3 shows the swivel and tower schematically;

FIG. 4 illustrates means for accommodating rotation between the towerand the riser; and

FIG. 5 is a view of the means of FIG. 4.

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION The general relation of a ship and its parts and a riserincorporating the principles of this invention are illustrated inFIG. 1. As illustrated in this embodiment an ocean going tanker I0 isprovided with internal storage tanks (not shown) for a large quantity ofcrude oil or the like. This tanker is in the same general form as oceangoing tankers with a pointed bow 11. The ship 10 is, however,essentially a barge since no substantial propulsion is provided onboard. Propulsion is not required since the ship is substantiallypermanently moored at an oil producing location and if moving isnecessary tugs are employed for propulsion. The pointed bow does,however, serve to minimize the resistance of the ship to environmentalforces such as wind, waves and currents. The ship-shaped hull of thestorage and production tanker provides excellent stability with a largecapacity for storing crude oil and provides acceptable motions inresponse to wave action.

Since the ship 10 remains on station in the oil producing field at alltimes, a permanent mooring system is provided which will accommodatehigh sea states. In order to obtain the lowest load on the mooringsystem in extreme sea states it is important to orient the ship in thedirection of least resistance to the environmental forces such as wind,waves and currents. There is, therefore, provided a mooring swivel 12for mooring the ship. As hereinafter described in greater detail themooring swivel 12 is mounted so as to rotate about a substantiallyvertical axis and the mooring swivel extends completely through theships hull from approximately the deck line through to the hull bottom.

In order to provide the maximum directional stability the mooring swivelI2 is located as far forward in the hull of the ship 10 as is possibleto allow the vessel to freely "weathervane" without providing auxiliarypropulsion to orient the ship. For automatic orientation the distancefrom the centerline of the mooring swivel 12 to the bow ll of the shipshould not exceed 20 percent of the ship's length measured from the bowto the stern. When the mooring swivel is located in the forward 20percent of the ship-shaped vessel the naturally occurring moments due towind, waves and current readily orient the ship in the direction forminimized forces without any auxiliary propulsion. If the swivel islocated aft of about 20 percent of the ship '5 length auxiliarypropulsion may be necessary to properly orient the ship. It will beapparent, of course, that if the mooring swivel is located substantiallyin the center of the ship that auxiliary propulsion is virtuallymandatory since little, if any, natural weathervaning can be expected.

The mooring swivel 12 is moored to the ocean floor 13 by a multiple leg,single point mooring system. In this system a plurality of mooringchains 14 are connected only to the swivel 12, as hereinafter describedin greater detail, and extend downwardly from the bottom of the mooringswivel and then outwardly in a radiating pattern to the ocean floor 13.In practice eight or more mooring chains are employed and extend forlong distances on the ocean floor, the drawing of FIGv i beingcompressed considerably for purposes of illustration. As its extremityeach of the mooring chains 14 is connected to a conventional anchor 16on the ocean floor. Additional details concerning the mooring system arecontained in copending U. S. Pat. application Ser. No. 838,435, filedJuly 2, 1969 entitled Permanent Ship Mooring System" by Bruno R.Naczkowski and assigned to North American Rockwell Corporation, assigneeof this invention, and which is hereby incorporated by reference forfull force and effect as if set forth in full herein. In the mooringsystem since the several anchor chains 14 of the mooring legs extend ina radiating pattern from the mooring swivel 12, this mooring swivel isheld in substantially a single location and is effectively preventedfrom rotating through any substantial degree. The substantially fixedmooring of the mooring swivel to the ocean floor serves to moor theentire ship I0, and since the swivel is essentially fixed in positionany weathervaning of the ship results in relative rotation about asubstantially vertical axis between the mooring swivel and the ship.

In order to provide communication with an underwater oil productionfacility a vertically extending riser I7 is connected to underwater oilproduction facilities 18 shown schematically in FIG. 1. A buoy 19 isconnected to the riser 17 at a point substantially below the watersurface so that the effect of wave action on the buoy l9 and riser isminimized. The buoy provides a substantial tension force on the riser tosupport the principle weight thereof. A fluid disconnect 21 is providedabove the buoy 19 so that the upper portion 22 of the riser can bedisconnected from the buoy if desired.

The upper end 22 of the riser passes through the mooring swivel I2 ashereinafter described and illustrated in greater detail and is connectedto a tower 23 above the deck of the ship. As is described in greaterdetail hereinafter means are provided on the tower for accommodatingrelative motion of the upper end 22 of the riser along the length of thetower 23 and for maintaining a substantially constant tension on theupper end of the riser. In addition, the tower is mounted on a pair oforthogonal gimbals (not shown in FIG. 1) so that the tower can tiltrelative to the ship and remain aligned with the riser during pitch androll of the ship and surge or displacement of the ship laterally from aposition directly over the underwater production facilities 18.

Additional details of the mooring swivel l2 and tower 23 are seen in theperspective cutaway view of FIG. 2 and principle features are set forthschematically in FIG. 3. As illustrated in FIG. 2 the mooring swivel 12is mounted in an aperture extending from the deck of the ship clearthrough to the bottom hull line of the ship. This permits the mooringchains 14 to radiate outwardly and downwardly from the ship's hull tothe anchor points without interference with any portion of the ship'shull which is therefore free to weathervane about the mooring swivel.This arrangement permits normal excursions or surge of the ship due towind, waves and currents without any danger of the ship overrunning anyportion of the mooring system and prevents any mutual interferencebetween the ship and the mooring system. As mentioned hereinabove theupper end 22 of the riser extends through the center of the mooringswivel and the riser is arranged in the center of the radiating anchorchains so that no mutual interference occurs between the riser and theanchor chains or between the riser and the ship.

The mooring swivel provided in this embodiment has an upper surface 24substantially flush with the deck of the ship so that many essentialservices can be provided directly between the deck and the upper surfaceof the mooring swivel. A circular array of heavy duty tapered rollerbearings 26 is provided between the upper flange surface 24 of themooring swivel and the main body of the ship 10. This bearing 26accommodates relative rotation between the mooring swivel and ship andcarries the principle downward mooring load from the mooring swivel tothe ship. The exterior portion of the mooring swivel then extendsdownwardly from the bearing 26 in a cylindrical portion 27 at least partway through the hull of the ship. The water line is normally in thiscylindrical portion, its position, of course, varying with the quantityof oil in the ship 's storage tanks. A pair of sliding bearing members29 and 31 are mounted on the ship and mooring swivel respectively nearthe bottom of the ship for accommodating transverse loads between themooring swivel and the ship which may be substantial when the forcesacting on the ship are high and must be reacted by the mooring chains.

The inner surface 32 of the mooring swivel is in the general form of anopen truncated cone having its smaller end near the upper surface 24 andflaring outwardly near the bottom of the ship. This provides clearancebetween the mooring swivel and the upper end 22 of the riser so that thepitch and roll of the ship can be accommodated without any impactbetween the riser and the swivel. As will become apparent hereinafterthere is no direct connection between the riser and the swivel.

A plurality of hawse tubes 33 extend between the upper surface 24 of theswivel and the bottom of the swivel. These hawse tubes 33 are gentlycurved to gradually change the direction of the anchor chains 14 whichare threaded therethrough as they extend between the bottom of themooring swivel and the deck of the ship. A single hawse tube is provided for each of the eight anchor chains provided in this embodimentand these hawse tubes are substantially equally spaced around andoutboard of the inner conical surface 32 of the mooring swivel. On theupper surface 24 of the mooring swivel adjacent the upper end of each ofthe hawse tubes 33 there is provided a chain stop 34 for individuallylocking each anchor chain l4 as is described in greater detail in theaforementioned copending patent application entitled, "Permanent ShipMooring System."

As mentioned hereinabove a tower 23 is supported above the deck of theship over the mooring swivel l2 and means are provided for accommodatingrelative motion between the tower and swivel. Thus, as provided in thisembodiment an outer gimbal ring 36 is rotatably supported for rotationabout an axis substantially normal to the swivel on the upper surface 24of the swivel by a circular array of tapered roller bearings 37. Theroller bearing 37 accommodates the weight of the tower and the tensionload applied to the upper end 22 of the riser as hereinafter describedin greater detail. The tower is prevented from rotating relative by theship by a forked member 40 (also seen in FIG. I, but omitted from FIG. 2for clarity) having its bifurcated end attached to the outer gimbal ring36 and its single end attached to the deck of the ship. This forkedmember thus prevents the gimbal ring 36 from rotating relative to theship about a substantially vertical axis and thereby prevents the towermounted within the gimbal ring from turning. Since the mooring swivel isseparated from the ship and tower by bearings 26 and 37, respectively,it is free to rotate relative to both about a vertical axis.

A pair of opposed trunnions 38 (only one is seen in FIG. 2) on the outergimbal ring 36 support an inner gimbal ring 39 which is therefore freeto tilt about an axis extending along the length of the ship. A pair oftrunnions 4! on the inner gimbal ring 39 are also connected to aspherical member 42 therewithin. This permits the spherical member totilt about an axis athwart the ship. The spherical member 42 and thetower 23 which is connected thereto is therefore free to tilt ontrunnions 38 and 41 about orthogonal axes and therefore in any directionrelative to the deck of the ship. The weight of the tower and tensionload on the upper end of the riser is transmitted through the sphericalmember 42 to the trunnions 41, thence to the inner gimbal ring 39 andthe trunnions 38 which apply the load to the outer gimbal ring 36mounted on the swivel plug 24 by the bearing 37. All of the loads areultimately carried to the buoyant hull of the ship through the heavybearing 26 supporting the swivel.

In order to maintain the gimbal mounted tower 23 in a substantiallyvertical orientation, four guying cables 43 are connected to the upperportion of the tower and extend outwardly therefrom to damping shockabsorbers 44 near the edge of the ship. Each of the shock absorbers ordampers 44 can be fixed in position, if desired, to maintain the towerin a fixed orientation relative to the deck of the ship. in normaloperation, however, the angular position of the tower follows that ofthe upper end of the riser and therefore the shock absorbers serve tolimit the angular excursions that the tower can follow and also serve toinhibit rapid or oscillatory motions.

The tower 23 comprises a structural base 46 which is rigidly connectedto the spherical member 42. Connected to the tower base 46 are a pair ofside rails 47 which extend substantially vertically along the length ofthe tower to form the principal structure of the tower. Each of the siderails has a cross section substantially in the form of an l-beam as canbe seen in the cutaway portion of FIG. 2. The side rails 47 are joinedtogether at the apex of the tower to provide a load carrying point forapplying a tension to the upper end of the riser.

A cagelike draw bar 48 is mounted with end portions fitted within theopposite sides between the flanges of the l-beam shaped side rails 47 ofthe tower. Bearing wheels 50 (FIG. 3) on the ends of the draw bar 48engage the web and flanges of side rails 47 of the tower to permitvertical motion of the draw bar along the length of the tower. Rollerchain sprockets 49 are mounted within each open side of each of theI-beam shaped side rails 47 and an endless roller chain 51 is providedbetween each of the four sets of sprockets 49. The roller chain 51 isconnected to the draw bar 48 so that vertical motion of the draw baralong the side rails moves the chain which causes the sprockets 49 torotate. The sprockets on opposite side rails of the tower areinterlocked so that the draw bar is constrained to move along the lengthof the tower without cocking. In addition the sprockets 49 are connectedto the impellers (not shown) of a conventional hydraulic damping systemwhich circulates hydraulic fluid to damp out small amplitude andoscillatory vertical motions of the draw bar relative to the tower.

The draw bar 48 is rigidly connected to a pair of vertical structuralmembers 52 extending above and below the draw bar on each side thereof.At their lower ends the structural members 52 are each connected to aring 53 which supports the upper end 22 of the riser by means of aconventional swivel bearing 55 (FIG. 3). Thus the tension load on theupper the open cage 54 thence through the clevis 56 and the hook $7 tothe cable block 58.

The tension load on the upper end of the riser is maintained constant bya conventional constant tension winch 59 on the deck of the ship. A pairof cables 61 extend from the winch 59 to the tower base and thence upalong the side rails 47 of the tower by way of a series of pulleys 62.The tension cables 6! are led through the block 58 as many times asdesired to obtain a given mechanical advantage. Thus by way of thecables 6| and the above-described structural linkages between the block58 and the upper end 22 of the riser a constant tension is applied tothe upper end of the riser. The nature of a constant tension winch is toapply a constant tension to the cable wound thereon irrespective of thecable position. Thus as the ship may rise or fall. for example. and theupper end 22 of the riser remains in a substantially fixed position, thedraw bar 48 will ride up and down the side rails of the tower and moreor less cable 6t is paid out or withdrawn as required by the core stanttension winch 59. With this arrangement a constant tension is applied tothe upper end of the riser throughout the extent of any excursion of theriser end along the length of the tower. Additional structural detailsof a tower and an alternative tower arrangement are contained incopending 1. .5. Pat. application Ser. No. 838.489. filed July 2. i969entitled. Underwater Riser and Ship Connection." and Ser. No. 729,286.entitled Marine Riser Structure" both by George W. Morgan. and assignedto North American Rockwell Corporation. assignee of this invention. theteachings of which are hereby incorporated by reference for full forceand effect as if set forth in full herein.

A typical riser useful in practice of this invention is described incopending U. 5. Pat. application. Ser. No. 72 l .0l4, entitled,Multiconduit Underwater Line by George W. Morgan. The riser described inthis copending up plication has a plurality of fluid conduits extendingalong the length for communicating fluids between the upper and lowerends of the riser. in addition. electrical conductors are provided alongthe length of the riser for carrying power and electrical signals.

In the practice of this invention the riser is substantially fixed inposition and the ship is free to pitch. roll, heave. and weathervaneabout the upper end of the riser and the riser and ship may surgetogether. As pointed out herelnabove the tower 23 rotates with the shipand the substantially fixed upper end of the riser is connected to thetower by way of a swivel hearing. In addition to this mechanicalconnection. electrical and fluid connections need be made to the upperend of the riser. Toward this end a conventional electrical wivel 63having slip rings or the like. is mounted on the riser supporting ring53. The electrical swivel is electrically connected to the conductors(not shown) of the riser. Flexible electrical cables 64 are alsoprovided between the tired portion at the electrical swivel 63 and thedeck of the ship ill for conducting power and electrical signalstherebetween.

A conventional fluid swivel 66 is mounted above the electrical swivel 63and in fluid communication with the conduits lnot shown) of the riser. Atypical fluid swivel useful in practice of this invention is describedin copentling U. 5. Pat. a plication Ser. No. 736.398. entitled,"Multiple Pipeline Swivel Connector by Eugene J. Camilla. assigned toNorth Amer icon Rockwell Corporation. astlanee of this lnvention. andwhich is hereby incorporated by reference for full force and effect asit set forth in full herein. Fluid connections are made from the severalmanifolds of the fluid swivel 66 in conven- 67ortheplpestheretols'emeebmiesllyeonneetedtotne draw bar48so thatthey areconstrainedtornoveupend downalongthelengthofthetowerwithmotionoftheriaerhead along the tower.Another complementary set of swivel joints arentedonthebaaeflol'thetowerandthcreforedo notm ewiththeriserhead.Asetofparallel rigidpipesflis connected to the upper swivel joints 67and a similar set of parallel rigid pipes is connected to the lowerswivel joints 68. The upper set of pipes 69 is connected to the lowerset of pipes I1 by a third set of conventional swivel joints 72. Theaxes of motion of the several swivel joints in the sets 67. 68 and 72are all parallel to that the sets of pipes 69 and 7i freely articulateas the upper end of the riser moves along the length of the tower. Thearticulated piping between the riser head moving along the length of thetower and the tower base provides for a great range of motion of theriser head without breaking fluid communication.

At the tower base the only motion encountered by the fluid lines is thetilt of the tower relative to the deck of the ship in two directions andthis motion is small enough that fluid connection between the lowerswivel joint 68 and the balance of the ship is made by conventionalflexible tubing 73.

As the ship pitches or rolls the substantially fixed upper end 22 of theriser causes the tower 23 to tilt relative to the ship. In order tominimize the stresses on the riser at the location of the tower gimbalmounts. i.e.. the spherical member 42. a riser guide or support tube 74is attached to the spherical member 42 and extends downwardly therefromsubstantially to the lower end of the mooring swivel 12. Thus both theguide tube 74 and the tower 23 are rigidly secured to the sphericalmember within the gimbal mounting. that is, the tower and guide tube areesentially a single rigid member pivotally mounted near its middle.Transverse loads between the riser and ship are coupled therebetween atthe lower end of the guide tube. at the spherical member. and at theupper end of the riser adjacent the draw bar 48. By so distributing theloads extreme bending moments on the riser at the gimbal support areavoided.

As the ship heaves, the riser must travel along the length of the guidetube 74 and therefore as illustrated in FIGS. 4 one 5 contoured rubberrollers 76 and 7! are rotatably mounted transverse to the tube axis atthe upper and lower ends respectively of the guide tube. The guiderollers 76 and 77. which may be segmented if desired to minimizefriction. are contoured so as to engage the periphery of the cylindricalriser and thereby permit iree relative motion of the riser along thelength of the tube while providing support. Transverse loads by theriser on the guide tube 74 are transmitted thereto by way ol'tbe rollers76 and 77.

It will also be apparent that ifthe guide tube 74 is lined relative tothe tower that the riser must rotate about an axis along the length ofthe guide tube. The contoured rubber rollers 7 and 77 hearing on theriser effectively prevent rotation between the riser and the guide tube74. Therefore a pair of bearings 78 are provided between the guide tube74 and the spherical member 42 so that the entire guide tube can rotateabout its axis. relative to the spherical member 42 and tower and yettransmit a substantial transverse load to the spherical member.Additional structural details of the guide tube 74 and its mounting arecontained in copending U. 5. Pat. application, Ser. No. 838,513entitled, "Riser Support Structure by John M. Deslierres. and assignedto North American Rockwell Corporation. assignee of this invention. theteachings of which are hereby incorporated by reference for full forceand elfect as if set forth in full herein.

In the nonnal course of operation of a ship and riser system as providedin practice of the principles of this invention the ship may pitch.roll. heave. yaw and drill or surge relative to the riser base at thesea floor. The meehanismt herelnabove described and illustratedschematically in FIG. 3 accommodate all of these motions withoutapplying undue stresses Thus. for example, as the ship heaven the risermoves relalive to the ship along the length of the guide tube 74extending through the mooring swivel l2. Rollers 76, 77 in the guidetube roll along the length of the riser to permit free relative motionin a substantially vertical direction. At the same time the uppermostend of the riser is connected to the draw bar 48 which is free to movealong the length of the tower 23 above the deck ofthe ship on rollers50. The draw bar and hence the upper end of the riser is supported fromthe top ofthe tower so that a constant tension is applied on the upperend of the riser by way of the cables 61 and the constant tension winch59 Vertical motion of the electrical connections is accommodated by theslack electric cables running to the upper end of the swivel from thedeck of the ship. Similarly, vertical mo tion of the fluid connectionsto the riser is accommodated by articulated piping between the riserhead and the base of the tower.

Pitch or roll of the ship changes the angular relation between the riserand the ship and this is also accommodated by the mechanisms hereinabovedescribed. As the riser tips relative to the ship (or vice versa)pivoting effectively occurs at the gimballed mounting of the tower baseabout the gimbal 41 and the transverse gimbal 38 (FIG. 2 and indicatedsche matically by the curved arrow at the top of the tower). As the Lriser tilts so does the tower to which it is attached with the llll' ingoccurring in the aforementioned gimbals. Moments are ap plied by theriser on the guide tube 74 and on the tower to cause tilting thereofandthe extent oftilt is limited by guy cables leading to a towerrestraining and damping system (not shown in FIG. 3). Tilt of the riser.and hence tower, is accom modated in the fluid connections by smallmotions of the air ticulnted piping and the flexible tubing extendingbetween the tower base and the ship.

Yawing or weathervaning of the ship relative to the riser may alsooccur. The riser is essentially fixed in position at its upper endrelative to the ship mooring system which com prises the mooring swivelpivotably mounted within the vertical aperture in the ship and theplurality of radially extending mooring chains for holding the ship in areasonably constant position. Thus the mooring swivel 12 issubstantially fixed in attitude relative to the ocean bottom by themooring chains and the riser is substantially fixed in attitude by itsinherent stiffness. Weathervaning of the ship relative to the mooringsystem is provided by the swivel plug which is mounted on bearings 26 topermit relative rotation. Weathervaning of the lower relative to theswivel is accommodated by bearings 37. Weathervaning of the shiprelative to the riser is provided by supporting the upper end of theriser by a swivel bearing 55 which permits the ship and tower to rotateabout the riser. in addition, bearing support 78 for the guide tube 74permits it to rotate with the riser relative to the ship so that nosubstantial torsion loads are applied to the riser by the ship. Rotationof the electrical line of the riser is accommodated by an electricalswivel at the upper end of the riser and a fluid swivel in the samegeneral location permits relative rotation of the fluid conduits of theriser relative to the ship.

Although the individual motions of heave. pitch, roll. yaw and surgehave been described it will be apparent that these may all occursimultaneously in a given situation; thus, for example, as the shipdrifts or surges ofi of a vertical position directly above the riserbase within the limits permitted by the mooring system, the riser willappear to tilt relative to-the ship and also be efiectively shorter. Atthe same time roll. pitch and heave oftlte ship due to wave action aredynamically accommodated by the mechanisms hereinabovedescribed. Duringthis period the ship may very well weathervane in response to tidalcurrent: so that all of the above-deseribed mptions are occurringsimultaneously.

"will be apparent that many modifications and variations of the presentinvention are possible in light of the above teaehings.-lli us, forexample. many variations canbe made in the detailed structures providingthe several degrees of freedom between the ship and riser while stillmaintaining constant tension on the riser plus fluid and electricalcommunicau'on between the ship and the riser during the complex motionsoccurring.

What is aimed is:

l. A sea going storage tanker comprising:

a streamlined hull:

means within said hull for storing substantial quantities of fluid;

said hull having a vertical aperture through said hull at a distancefrom the bow of no more than twenty percent of the bow to stem length ofsaid hull;

a mooring plug in said aperture;

means for anchoring said mooring plug to the sea floor;

bearing means between said mooring plug and said hull for permittingrelative rotation therebetween about a sub stantially vertical axis;

said mooring plug having a substantially vertical passage;

21 substantially ver1ically disposed tower mounted over the passage insaid mooring plug and connected to said hull for minimizing rotationbetween said tower and said hull about a substantially vertical axis;

support means for said tower for permitting limited rotation ofsaidtower relative to said hull about at least two axes in a substantiallyhorizontal plane means on said tower for connection to an upper end ofan underwater riser. and means for connection further comprising:

means for applying a substantially constant tension to the end of theriser during relative translation of the riser end along the height ofthe tower;

a fluid manifold mounted for translation along the length of said towerand restricted from rotation relative to said tower abouLan axis alongthe length thereof, said fluid manifold being connected to said riserfor transfer of fluids therebetween; and

an articulated fluid conduit between said fluid manifold and the tower,

2, A tanker as defined in claim I wherein said support means formounting said tower comprises:

a tower base;

first and second orthogonally arranged gimbal means on said mooring plugand supporting the lower end of said tower for limited rotation thereofabout at least two axes in a horizontal plane;

a plurality of guy cables connected to the upper end of said tower; and

constant tension means interconnecting each of said guy cables and saidhull for permitting limited motion of the upper end ofsaid towerrelative to said hull.

3, A combination comprising:

a floating structure at the surface of the sea;

an elongated multiple conduit riser having a lower end connected to thesea floor;

means interconnecting the floating structure and the upper end of saidriser, said means comprising means connected to the upper end of saidriser for maintaining a substan tially constant tension on said riserdespite relative mo tion between said surface structure and the seafloor;

means between the ends of said riser for conveying fluids between theinterior of each of the multiple conduits and said floating structurecomprising:

' a fluid manifold surrounding a portion of the length of said riser andin fluid communication with a conduit thereof, and

means for accommodating relative rotation between said manifold and saidconduit about an axis substantially coincident with the axis of saidriser so that said surface structure may rotate about said riser; and

means at a location substantially coincident with the upper end of said;riser for mooring the floating structure to the sea floor for limitinglateral translation thereof comprising:

a mooring plug including a circular portion,

means for anchoring the mooring plug to the sea floor. and

axis;

means for mounting said tower on said floating structure for permittinglimited rotation therebetween about at least a pair of axes in asubstantially horizontal plane;

said tower supporting at least a portion of said means for applyingconstant tension to said riser; and

means on said tower for fluidly interconnecting the fluid manifold onsaid conduit and a point adjacent the base of said tower.

1. A sea going storage tanker comprising: a streamlined hull: meanswithin said hull for storing substantial quantities of fluid; said hullhaving a vertical aperture through said hull at a distance from the bowof no more than twenty percent of the bow to stern length of said hull;a mooring plug in said aperture; means for anchoring said mooring plugtO the sea floor; bearing means between said mooring plug and said hullfor permitting relative rotation therebetween about a substantiallyvertical axis; said mooring plug having a substantially verticalpassage; a substantially vertically disposed tower mounted over thepassage in said mooring plug and connected to said hull for minimizingrotation between said tower and said hull about a substantially verticalaxis; support means for said tower for permitting limited rotation ofsaid tower relative to said hull about at least two axes in asubstantially horizontal plane; means on said tower for connection to anupper end of an underwater riser, and means for connection furthercomprising: means for applying a substantially constant tension to theend of the riser during relative translation of the riser end along theheight of the tower; a fluid manifold mounted for translation along thelength of said tower and restricted from rotation relative to said towerabout an axis along the length thereof, said fluid manifold beingconnected to said riser for transfer of fluids therebetween; and anarticulated fluid conduit between said fluid manifold and the tower. 2.A tanker as defined in claim 1 wherein said support means for mountingsaid tower comprises: a tower base; first and second orthogonallyarranged gimbal means on said mooring plug and supporting the lower endof said tower for limited rotation thereof about at least two axes in ahorizontal plane; a plurality of guy cables connected to the upper endof said tower; and constant tension means interconnecting each of saidguy cables and said hull for permitting limited motion of the upper endof said tower relative to said hull.
 3. A combination comprising: afloating structure at the surface of the sea; an elongated multipleconduit riser having a lower end connected to the sea floor; meansinterconnecting the floating structure and the upper end of said riser,said means comprising means connected to the upper end of said riser formaintaining a substantially constant tension on said riser despiterelative motion between said surface structure and the sea floor; meansbetween the ends of said riser for conveying fluids between the interiorof each of the multiple conduits and said floating structure comprising:a fluid manifold surrounding a portion of the length of said riser andin fluid communication with a conduit thereof, and means foraccommodating relative rotation between said manifold and said conduitabout an axis substantially coincident with the axis of said riser sothat said surface structure may rotate about said riser; and means at alocation substantially coincident with the upper end of said riser formooring the floating structure to the sea floor for limiting lateraltranslation thereof comprising: a mooring plug including a circularportion, means for anchoring the mooring plug to the sea floor, andbearing means interconnecting the circular portion of said mooring plugand said floating structure for permitting relative rotationtherebetween for minimizing relative rotation between said riser andsaid mooring plug while permitting relative rotation between saidfloating structure and said riser.
 4. A combination as defined in claim3 wherein said means interconnecting the floating structure and theriser comprises: a tower mounted on said floating structure over saidmooring plug to the floating structure so as to prevent relativerotation therebetween about an approximately vertical axis; means formounting said tower on said floating structure for permitting limitedrotation therebetween about at least a pair of axes in a substantiallyhorizontal plane; said tower supporting at least a portion of said meansfor applying constant tension to said riser; and means on said tower forfluidly interconnecting the fluid manifold on said conduit and a pointaDjacent the base of said tower.